The present invention relates to a water-lubricated screw compressor where the water-supply to resin rotors is made possible at the screw compressor's start time.
In a water-lubricated screw compressor, water is injected into a compression chamber that is formed of a casing and a single pair of male/female screw rotors. This feature allows the water-lubricated screw compressor to acquire clean air as an oil-free compressor, and to be superior in its cooling effect and seal effect as compared with those of conventionally-used dry screw compressors. As a result, the water-lubricated screw compressor is capable of implementing its low discharged-air temperature, small revolution number, and high performance.
On account of these characteristics, the water-lubricated screw compressor is expected to prevail in market from now on. The water injection into the compression chamber, however, requires that rust of the casing and rotors be prevented. High rust-resistant bronze is often used for the casing. Similarly, the high rust-resistant bronze is sometimes used for the rotors. In the bronze-formed rotors, however, the lubrication between the rotors is difficult to implement. Accordingly, non-contact driving between the rotors is implemented by setting up timing gears. Meanwhile, a resin, whose water lubrication is satisfying enough, is used for the rotors. This resin-used scheme makes it possible to implement directly contact driving between the rotors, thereby making it unnecessary to set up the timing gears.
The directly contact driving using these resin-formed rotors makes it possible to shorten the inter-rotors clearance, thereby allowing an enhancement in the compressor's performance. Also, setting up none of the timing gears also makes unnecessary an oil-lubricated mechanism of the timing gears. This condition simplifies the structure surrounding each bearing chamber.
Nevertheless, when these resin rotors are used, the following drawback exists: First, during the compressor's operation, the air pressure inside a water separator is present. Here, the water separator serves as a water tank as well. At this time, the air pressure inside the compression chamber of a water-supply unit of the compressor is lower than the air pressure inside the water separator. Accordingly, the water inside the water tank is supplied into the compression chamber by the pressure difference therebetween. At the compressor's start time, however, the air pressure inside the water separator absent. Consequently, no water is supplied into the compression chamber, until the air pressure inside the water separator becomes higher than the air pressure inside the compression chamber after the compressor's operation is started. As a result, at the worst, the resin rotors rotate in a state of remaining dried without the lubrication therebetween. Usually, it takes 5 to 10 seconds until the discharge pressure of the water-lubricated screw compressor rises up to its rated pressure, i.e., 0.7 MPa.
Also, when a roller bearing is used as the bearing of each of the rotors, and when the oil lubrication by the splash is implemented, a lip seal for sealing each rotor axis is used on the compression-chamber side of each bearing chamber of each rotor. This lip seal is used in order to prevent the oil from leaking from the bearing-chamber side onto the compression-chamber side. Moreover, a lip seal is used at the inlet-side end portion of the compression chamber of each rotor axis. This lip seal is used in order to prevent the water, which is injected into the compression chamber, from leaking from the compression-chamber side onto the bearing-chamber side. Furthermore, at the discharge-side end portion of the compression chamber, a mechanical seal is used. This mechanical seal is used, because the pressure difference is significant between the compression chamber and each bearing chamber. On account of this situation, during the compressor's operation, the water is also supplied to a slide part of the fixed member and rotational member of the mechanical seal in order to implement its lubrication and cooling. This water-supply is performed by the pressure difference between the water separator and the water-supply unit of the mechanical seal.
At the compressor's start time, however, the air pressure inside the water separator absent as is the case with the clearance between the resin rotors. Accordingly, the water-supply is not performed until the air pressure inside the water separator is caused to rise by the discharged air. This situation requires that a water-injecting method which is different from the water-supply from the water tank inside the water separator be provided at the compressor's start time. Here, of course, this method is required in order to perform the water-supply to the water-supply unit of each resin rotor and the slide part of the mechanical seal.
As a method for supplying the water to the water-supply unit of the water-lubricated screw compressor at the compressor's start time, for example, the following method is disclosed in JP-A-2000-45947: Namely, in this method, the water-supply is performed to the inter-resin-rotors clearance and the slide part of the mechanical-seal unit by using an external pressure-added water-supply line, and opening/closing a solenoid valve set up in this water-supply line.